- Jan 15, 2022
- Reaction score
So I have a SL 100 on the bench and after going through it I connected my bias meter to the two inside first then, the two outside 6550s. And I'm ony able to adjust the bias from 0 - 25 mA , acording to my chart Sovtek 6550s are 35W and my calculations tell me between 38mA (min) and 54mA (max). Anyone ever see this situation? Where I have the bias currently set now aprox 19ma -23mA it plays and sounds pretty good for no mods, which has me baffled beause you would expect the tubes (valves) to be starving and sputtering out. I've checked some of the voltages with no signal input and have 460v on the plate, 457 screen, -59.7mV on the control grid, 0v on the cathode. I suspected maybe one of the two 10uF 160v caps that set up the neg voltage for the bias circuit because I tested both with an ESR meter and the one tested a bit low. I'll have to wait til Monday to get a couple caps from the local amp supply house. Also, one difference I notice that in the control grid\bias node there is 220k resistors that split off to both sections. The schematic I have for the 1959 MKII with 6550 tubes shows what looks like 82k but, it appears this amp may have had the 220k from the beginning since they don't look like they have ever been replaced. Somethinfs robbing the bias current? Not sure what I'm missing, Thanks in advanced for any assistance.